Chlorine-free bleaching agents have long been used for bleaching mechanical pulps. In recent years, it has become increasingly common to bleach also chemical pulps with chlorine-free bleaching agents, such as hydrogen peroxide and ozone, even in the first stages. It has been considered necessary to pretreat the pulp directly after digestion and an optional oxygen-delignifying stage so as to avoid deteriorated pulp properties and an excessive consumption of the bleaching agent. Pretreatment of the pulp primarily involves acid treatment and treatment with a complexing agent or salts of alkaline-earth metals, optionally in combination. Strongly acid pretreatment removes desirable as well as undesirable metal ions from the original positions in the pulp. Treatment with suitable complexing agents primarily removes the undesirable metal ions, while the desirable ones are largely retained. Treatment with salts of alkaline-earth metals maintains or reintroduces the desirable metal ions.
EP-A-0 402 335 thus discloses the pretreatment of chemical pulp with a complexing agent directly after digestion or oxygen delignification, to make a subsequent alkaline peroxide bleaching more efficient.
EP-A-0 480 469 relates to delignification of lignocellulose-containing pulp with oxygen. The pulp can be delignified or bleached before or after the oxygen stage with peroxide-containing compounds, such as hydrogen peroxide or peracetic acid, chlorine dioxide and/or ozone. Use of sequences with both peracetic acid and hydrogen peroxide, results in a significant decrease in pulp viscosity.
U.S. Pat. No. 5,091,054 describes a process where a pulp is treated with a sequence in two steps. In the first step peroxomonosulphuric acid, i.e., Caro's acid (=an inorganic acid containing sulphur), is added. A complexing agent may be added in the treatment with Caro's acid. In the second step the pulp is bleached with peroxide and/or oxygen.
With increasingly stringent environmental standards, there is a growing need for completely chlorine-free processes for delignifying and bleaching lignocellulose-containing pulps. To produce fully bleached pulps with unaltered strength properties in a reasonable number of stages and with a reasonable consumption of bleaching agents, it has become necessary to consider using also powerful, and hence difficultly-controlled, bleaching agents having a high delignifying and/or bleaching capacity.